Process for the oxidation of di-, tri-, Oligo- and polysaccharides into polyhydroxycarboxylic acids, catalyst used and products thus obtained

ABSTRACT

Process for the selective oxidation of di-, tri-, oligo- and polysaccharides comprising a reducing terminal function of the aldose type into polyhydroxycarboxylic acids, wherein the oxidation is carried out in an alkaline medium by means of an oxygen-containing gas, in the presence of a noble metal based catalyst selected from the group constituted by palladium, platinum, rhodium and osmium and fixed on an inert support, said catalyst being &#34;doped&#34; with one or several metals, or promoters, of Groups IV, V or VI of the Periodic Table.

This invention essentially relates to a process for the oxidation ofdi-, tri-, oligo- and polysaccharides into polyhydroxycarboxylic acids.

It is known to prepare polyhydroxycarboxylic acids by oxidizing thecorresponding polysaccharides using electrochemical techniques in thepresence of CaBr₂ ; it has also been proposed to oxidize the freealdehyde functions of the polysaccharides with sodium hypobromite orhypochlorite; finally, the catalytic oxidation of said polysaccharideshas been proposed.

The aforesaid electrochemical processes, besides being complex, entailmany drawbacks, especially as regards the purification of the productsthus obtained and the removal of bromine following the oxidationreaction. They therefore, do not prove economically profitable on anindustrial scale.

The techniques of oxidation of the free aldehyde functions by means ofNa-hypobromite or hypochlorite are not selective and, actually, bringabout breakage of the glucosidic bonds along with a generation ofoxidized products having a low molecular mass.

Catalytic oxidation using palladium or platinum based catalysts, fixedon carbon has been disclosed in French patent No. 70 18091. Theseprocesses lack selectivity and not only lead to low conversion rates,but furthermore, do not make it possible to avoid over-oxidationphenomena, especially in the case of platinum.

The main purpose of the present invention therefore is to remedy thesedrawbacks and to provide a process for the selective oxidation of thepolysaccharides, thereby better meeting the various prerequisites of thepractice in comparison to the methods already available.

Now, Applicants have found that, surprisingly and unexpectedly, it waspossible, in an alkaline medium, to selectively oxidize underadvantageous economical conditions, the terminal aldehyde functions ofthe polysaccharides into carboxylic functions, without any degradationof the initial polymeric structure and without the occurrence of anysecondary reaction associated with the disruption of the glucosidicbonds, when using catalysts based on noble metals of the groupconsisting of palladium, platinum, rhodium and osmium, fixed on an inertsupport and "doped" with one or several of the metals of Groups VI, V orVI of the Periodic Table or system, which constitute the "promoters".

Consequently, the process according to the invention for the preparationof polyhydroxycarboxylic acids is essentially characterized by the factthat there are oxidized one or several polysaccharides in an alkalinemedium and, by means of an oxygen-containing gas, in the presence of acatalyst based on a noble metal of the group consisting of palladium,platinum, rhodium and osmium, fixed on an inert support and "doped" withone or several of the metals of Groups IV, V or VI of the Periodic Tableor system.

In addition to this process of preparation of polyhydroxycarboxylicacids, another object of the invention is use of the aforesaid catalystsin connection with their application to said process as well as thepolyhydroxycarboxylic acids obtained by using same.

The polysaccharides, whose selective oxidation is made possible by meansof the process in accordance with the invention comprise:

disaccharides having a reducing function of the aldose type such aslactose, maltose, isomaltose, cellobiose, xylobiose and mannobiose, aswell as

trisaccharides, oligosaccharides and products resulting from thehydrolysis of starch cellulose and hemicelluloses containing a reducingterminal function of the aldose type and a mixture of thesepolysaccharides.

The starch hydrolysis is generally carried out by the acid and/orenzymatic way and leads to the production of glucose syrups. Amongproducts from the hydrolysis of hemicelluloses, there may be mentionedthe D-galacto-D-mannans, the D-gluco-D-mannans, the L-arabino-D-xylans,and the D-xylo-L-arabinans.

The molecular mass of the polysaccharides does not constitute alimitation to the invention, provided of course that the products to beoxidized are water-soluble. However, the polysaccharides with a highmolecular mass have high viscosities in aqueous solution, so that itbecomes necessary to proceed with low concentrations, which does notprove commercially advantageous.

In the case of use of the process in accordance with the invention inthe oxidation of products resulting from the starch hydrolysis, themixture of polysaccharides resulting from the hydrolysis ischaracterized by its reducing power or D.E. (dextrose-equivalent) and bythe glucidic distribution or spectrum; the process according to theinvention can be carried out on any starch hydrolysate or glucose syrupwhose DE is comprised between 90 and 5 and, preferably, between 85 and15 and, still more preferably, between 75 and 15.

The lower limit of the DE is imposed, on the one hand, by viscosity andsolubility problems as mentioned above, and, on the other hand, by thekinetic which decreases rapidly decreasing with the degree ofpolymerization or DP.

The catalysts based on noble metal, especially those based on palladiumand/or platinum, are known per se; the "support" generally consists offinely divided carbon, alumina, silica, barium sulfate or titaniumoxide; carbon on the one hand, and Pd and Pt on the other hand, beingpreferred.

The Applicants have found out that, surprisingly, the presence in thesecatalysts of one or several of the aforesaid promoters makes it possibleto decisively increase the kinetic, yield and selectivity of theoxidation reactions, in an alkaline medium, of the polysaccharides intopolyhydroxycarboxylic acids.

The incorporation of the aforesaid promoters into the catalyst can becarried out before or after having deposited the noble metal onto theinert support or simultaneously with this depositing operation.

It is also possible to introduce the promoter in solution into areaction medium containing the polysaccharides in aqueous solution aswell as a catalyst based on noble metal. In this case, the deposit ofthe promoter is performed in situ in the reaction medium.

Preferably, the promoters are used in the form of salts in order tofacilitate their solubilization in an aqueous, generally acid medium.

For the preparation of the "doped" catalyst, the solution of thepromoter salts is mixed with an aqueous suspension of the catalystconsisting of the noble metal, the impregnation of said catalyst withthe promoter in the form of salt being effected by maintaining themixture under stirring for a duration of at least a few seconds toseveral hours, generally between 15 minutes and 2 hours.

The suspension thus impregnated with the supported catalyst based onnoble metal is then made alkaline by adding a base such as NaOH, KOH,sodium carbonate and other, before the completion of the step ofreduction of the promoter which can be performed at a temperaturebetween 20° C. and 100° C. by means of chemical reducing agents such asformalin, sodium formate, sodium boron hydride, hypophosphorous acid,hydrazine, glucose or other reducing sugars.

The catalyst thus reduced is filtered, washed, dried or used as such.

It should be pointed out that the reduction of the catalyst can beperformed within the reaction medium of the catalytic oxidation processsince the latter initially takes place in the presence of reducingpolysaccharides and in an alkaline medium.

Preferably, a catalyst obtained by the addition of the promotersubsequently to the noble metal deposit onto the inert support is used;the promoter can also be incorporated into a commercially availablecatalyst based on noble metals on an inert support.

The content of promoters in the final catalyst, expressed in terms ofmetal, is generally comprised between 1 and 300 wt % with respect to thenoble metal.

Still preferably, as promoter, bismuth, lead, antimony, tin or seleniumand, most particularly preferred, bismuth and lead are used.

Consequently, the catalysts preferred in connection with the processaccording to the present invention are those obtained by depositingbismuth and/or lead onto a catalyst based on palladium and/or platinum,supported on carbon.

The palladium and/or platinum content expressed in terms of metal isgenerally comprised between 1 and 10 wt % with respect to the support.

The bismuth and/or lead content expressed in terms of metal is comprisedbetween 1 and 300 wt % with respect to palladium and/or platinum,preferably between 5 and 100%.

This being said, the process in accordance with this invention for thepreparation of polyhydroxycarboxylic acids and of their salts comprises:

the introduction of an aqueous solution of at least one polysaccharideinto a reaction vessel equipped with a stirring device, said solutionhaving a concentration of polysaccharides preferably comprised between 5and 60 wt %, the lower limit being imposed by a concern forprofitability and the upper limit by the solubility of oxygen in highlyviscous media, and the risk of crystallization of the salts of aldonicacids formed during the reaction, the oxidation of a glucose syrupbeing, for example, preferably performed at a concentration comprisedbetween 20 and 40 wt %,

the dispersion into this solution of the catalyst used according to theinvention, the quantity of catalyst introduced being such that thequantity of palladium and/or platinum, expressed in terms of metal, iscomprised between 0.005 and 1 wt % with respect to the polysaccharidesand, preferably, between 0.01 and 0.4%,

the starting of the reaction by the simultaneous supply of a flow of airor oxygen-containing gas and of an alkaline agent, the reactiontemperature generally ranging between 20° and 90° C., preferably between25° and 60° C. for a reaction time comprised between 30 minutes and 5hours,

the introduction into the reaction medium of alkaline agents with a viewto neutralizing the polyhydroxycarboxylic acids formed, in order tomaintain a constant catalytic activity during the reaction and, moreprecisely, in order to keep the pH of the reaction medium at a valuesufficient to ensure the desorption of the polyhydroxycarboxylic acidsformed and to avoid the over-oxidation of same, without however, havingthis pH reaching values likely to promote reactions of isomerization ofaldose into ketose, said pH being maintained in the practice at a valuecomprised between 7.5 and 11.0, preferably between 8.0 and 10.0.

The alkaline agent is selected from the group comprising sodiumhydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide,magnesium hydroxide, depending on the goal to be reached; thus, sodiumhydroxyde will be used in order to obtain sodium salt from thepolyhydroxycarboxylic acid corresponding to the polysaccharide used;zinc or manganese carbonate can also be used as well as any other zincor manganese salts for which the corresponding hydroxides are obtainedin situ by adding an alkaline agent such as sodium hydroxide orpotassium hydroxide.

The catalysts used according to the invention and obtained by depositinga promoter, particularly the palladium-bismuth or palladium-leadcatalysts, supported on carbon, have a catalytic activity which ispractically independent from the degree of polymerization of thepolysaccharides subjected to oxidation, so that the speed of reactionremains practically constant when mixtures of polysaccharides such asproducts resulting from the starch hydrolysis are oxidized; in such acase, no reaction of over-oxidation or degradation of thepolysaccharides is observed.

The rate of conversion of the polysaccharides subjected to the processaccording to the invention is higher than 90% and more particularlycomprised between 95 and 100%.

These remarkable performances are all the more exceptional since theyare preserved even in the case of an important number of recyclings ofthe catalysts used in accordance with the invention.

The catalysts which are preferably used, i.e. those wherein the promoteris deposited after having deposited the noble metal onto the support areeasily prepared and show a high stability and can be subjected toregeneration by depositing a new promoter charge, the required quantityof promoter being, anyhow, always low since the deposit is exclusivelycarried out on the surface of the noble metal.

The oxidized polysaccharides, obtained by use of the process accordingto the invention can be used in many fields, especially in sodium saltform, as chelating or complexing agents, for the cleaning of glass ormetal articles or items, especially of iron or aluminum, as additivesfor detergents, or in the field of hydraulic binders as a fluidifyingagent reducing water, as concrete retarding admixtures, etc.

They can also be used in the pharmaceutical field, as for example,calcium lactobionate.

The invention will be best understood by means of the followingnon-limiting examples which describe among other things advantageousembodiments of the invention.

There is first cited a few examples for the preparation of the catalystused in the process according to the invention.

EXAMPLE 1

Preparation of a catalyst containing 5% of Pd and 3.5% of Bi on carbon,by depositing bismuth onto a palladium on carbon or Pd/C catalystcommercially available.

A quantity of 6 g of dry Pd/C catalyst commercially available (DEGUSSA198 R/W with 5% Pd) is suspended in 80 ml of distilled water acidifiedwith 1 ml of concentrated hydrochloric acid (37% HCl). To thissuspension, there is added a solution consisting of 0.3 g of bismuthsubnitrate dissolved in a mixture of 2 ml of concentrated hydrochloricacid and 5 ml of distilled water.

After stirring for two hours, there are introduced 4 g of caustic sodiumhydroxide in solution in 30 ml water. The mixture is brought to atemperature of 40°-50° C. for 4 hours, then 1.5 ml of formalin (37-38%aqueous solution) is added. The mixture is brought to 85° C. for 1 hour.The catalyst thus obtained is filtered and washed.

EXAMPLE 2

Preparation of a catalyst containing 5% of Pd and 3.5% of Bi on carbonby depositing bismuth prior to depositing palladium.

A quantity of 6 g of dry active carbon is suspended in 80 ml ofdistilled water. To this suspension, there is added a solutionconsisting of 0.3 g of bismuth subnitrate dissolved in a mixture of 3 mlof concentrated hydrochloric acid and 5 ml of distilled water.

Stirring is continued for 6 hours in order to have the bismuthcompletely adsorbed on the active carbon. Then, a solution of 0.5 g ofpalladium chloride (0.3 g of metal palladium) in 1.5 ml of hydrochloricacid and 5 ml of distilled water is added. 4 g of caustic NaOH insolution in 30 ml of water are added and the mixture is brought to atemperature of 40° C. for 5 hours. After having added 1.5 ml of a 37%formalin solution, the suspension is maintained at 85° C. for one hour.The catalyst thus obtained is filtered and washed.

EXAMPLE 3

Preparation of a catalyst containing 5% of Pd and 2.5% of Pb on carbonby depositing the lead onto a Pd/C catalyst commercially available.

A quantity of 6 g of dry Pd/C catalyst commercially available (DEGUSSA198 R/W with 5% Pd) is suspended in 80 ml of distilled water. To thissuspension, 20 ml of an aqueous solution containing 0.3 g of leadacetate are added. The lead is caused to be absorbed for one hour understirring. 30 ml of an aqueous solution containing 4 g of Na₂ CO₃ areadded and the mixture obtained is brought to a temperature at 40° C. forfour hours. 1.5 ml of formalin is added and the suspension is maintainedat 85° C. for 1 hour. The catalyst thus obtained is then filtered andwashed with distilled water.

There is now described a few examples of the use of the processaccording to this invention.

EXAMPLE 4

Preparation of sodium maltobionate by oxidation of maltose.

Four experiments are carried out by introducing each time individuallyinto a reaction vessel having a capacity of 1 l, equipped with astirring device and a thermometer, a sintered rod for the blowing ofair, an electrode and a continuous introduction device, a quantity of666 g of an aqueous maltose solution with 30% of solid matters as wellas a quantity of 6 g of respectively each of the dry catalysts accordingto examples 1 to 3 for the first three experiments (experiments a, b andc) and a quantity of 6 g of the dry catalyst commercially available,used in examples 1 and 3 for the fourth experiment (experiment d).

The reaction is effected at 35° C. and air is blown while simultaneouslyintroducing a 30% aqueous solution of NaOH in order to maintain the pHat a value of 9.0±0.5.

The reaction is stopped when the theoretical quantity of sodiumhydroxide has been consumed, which gives the reaction speed; thereaction product is then separated by filtration and the percentage ofresidual reducing sugars is determined, which makes it possible tocalculate the rate of conversion of the substrate or feedstock.

The maltose used in experiment c is a maltose having a purity of 99.9%and the one used in the experiments a, b and d has 95% purity.

In table I are recorded the results from the above-mentionedmeasurements and determinations.

                  TABLE I                                                         ______________________________________                                        Experi-              Reaction Reducing                                                                             Maltose                                  ment  Catalyst       time     sugars conversion                               Nr    used           (hr)     (%)    rate (%)                                 ______________________________________                                        a     5% Pd and 3.5% 1.40     1.3    97.5                                           Bi on carbon                                                                  (example 1)                                                             b     5% Pd and 3.5% 1.50     1.4    97.0                                           Bi on carbon                                                                  (example 2)                                                             c     5% Pd and 3.5% 0.40     0.6    98.9                                           Bi on carbon                                                                  (example 1)                                                             d     5% Pd/C commer-                                                                              4.40     2.6    95.1                                           cially available                                                              (DEGUSSA 198 R/W)                                                       ______________________________________                                    

EXAMPLE 5

Preparation of sodium lactobionate by oxidation of lactose.

The operating procedure adopted to carry out this example is identicalto the one described in example 4, except for the quantity of rawmaterial implemented which now amounts to 1000 g of solution with 20 wt% of lactose having a purity of 99.8%.

The catalysts used are, on the one hand, the one of example 1 and, onthe other hand, a Pd/C catalyst commercially available (DEGUSSA C 101R/W with 5% Pd).

The numeric data of example 5 are recorded in table II.

                  TABLE II                                                        ______________________________________                                                      Reaction  Reducing Lactose                                                    time      sugars   conversion                                   Catalyst used (hr)      (%)      rate (%)                                     ______________________________________                                        5% Pd and 3.5% Bi                                                                           1.20      4.6      91                                           on carbon                                                                     (example 1)                                                                   ______________________________________                                    

In the case of the catalyst commercially available and after four hours'reaction time, the quantity of sodium hydroxide consumed was lower than10% with respect to the theoretical quantity. Under these conditions,the test was stopped.

EXAMPLE 6

Preparation of polyhydroxycarboxylic acids by oxidation ofpolysaccharides.

A series of 12 experiments is carried out with, as raw material, eightproducts resulting from the hydrolysis of starch whose the DE rangesbetween 90 and 27.7.

The operating procedure adopted for the oxidation of the raw materialsidentified in table III is thoroughly identical with the one indicatedfor example 4.

The catalysts are either those according to examples 1 to 3, or thecatalyst commercially available used in these examples 1 and 3.

Still according to the same method, there are determined the reactiontime and, on the finished product, the percentage of residual reducingsugars which makes it possible to calculate the conversion rate of thepolysaccharides.

                                      TABLE III                                   __________________________________________________________________________    Experi-      DE    Glucidic spectrum                                          ment                                                                              Definition of                                                                          of the                                                                              glu-                                                                             mal-                                                    Nr  the hydrolysate                                                                        hydrolysate                                                                         cose                                                                             tose                                                                             DP3                                                                              DP4                                                                              DP5                                                                              DP6                                                                              DP7                                                                              DP8                                                                              DP9                                                                              DP10-20                                                                            DP20                       __________________________________________________________________________    e   Hydrolysate with                                                                       90    85.4                                                                              9.4                                                                             3.9            2                                         high DE                                                                   f   Hydrolysate with                                                                       84    71.5                                                                             14.0                                                                             4.2            6.1                                       medium DE *                                                               g   Maltose-rich                                                                           65.8  40.2                                                                             28.3                                                                             10.2                                                                             1.6                                                                              0.8                                                                              0.8                                                                              1.2                                                                              1.7                                                                              2.4                                                                               8.7 4.0                            glucose syrup                                                             h   Glucose syrup                                                                          40.8  17.1                                                                             14.6                                                                             10.7                                                                             8.1                                                                              6.5                                                                              5.0                                                                              4.5                                                                              4.5                                                                              3.3                                                                              20.6 5.0                            with high DE                                                              i   Glucose syrup                                                                          38    14.2                                                                             11.4                                                                             9.8                                                                              8.8                                                                              7.8                                                                              5.7                                                                              5.1                                                                              4.5                                                                              3.9                                                                              23.0 5.9                            with medium DE                                                            j   Glucose syrup                                                                          33.1  11.4                                                                              9.6                                                                             8.4                                                                              7.5                                                                              6.7                                                                              5.1                                                                              4.8                                                                              4.4                                                                              4.0                                                                              27.3 10.9                           with low DE                                                               k   Glucose syrup                                                                          39.8  13.1                                                                             13 13 7.7                                                                              10.8                                                                             11.5                                                                             2.8                                                                              2.7                                                                              2.7                                                                              17.5 4.8                            with low content                                                              in polysaccharides                                                        m   Maltodextrine                                                                          27.7   9.3                                                                              7.7                                                                             7.7                                                                              7.8                                                                              6.6                                                                              5.9                                                                              5.1                                                                              4.7                                                                              4.2                                                                              29.2 11.2                           with low DE                                                               __________________________________________________________________________     * fructose content 4.1%                                                  

In table IV, the raw material and the catalyst used have been identifiedfor the twelve experiments and the result of the aforesaid determinationhas each time been indicated.

                                      TABLE IV                                    __________________________________________________________________________                                    Conversion                                                          Reaction                                                                           Reducing                                                                           rate of the                                                         time sugars                                                                             polysac-                                      Raw material                                                                           DE Catalyst  (hr) (in %)                                                                             charides (%)                                  __________________________________________________________________________    Hydrolysate with                                                                       90.0                                                                             Pd/Bi/C according                                                                       2.40 1.6  97.9                                          high DE (e) to example 1                                                      Hydrolysate with                                                                       84.0                                                                             Pd/Bi/C according                                                                       2.30 6.8* 97.0                                          medium DE (f)                                                                             to example 1                                                      Maltose-rich                                                                           65.8                                                                             Pd/Bi/C according                                                                       4.30 1.70 97.5                                          glucose syrup (g)                                                                         to example 1                                                      Maltose-rich                                                                           65.8                                                                             Pd/Bi/C according                                                                       4.20 1.80 97.2                                          glucose syrup (g)                                                                         to example 2                                                      Maltose-rich                                                                           65.8                                                                             Pd/Bi/C according                                                                       4.30 1.80 97.2                                          glucose syrup (g)                                                                         to example 3                                                      Maltose-rich                                                                           65.8                                                                             Pd/C commercially                                                                       7.30 11.40                                                                              82.7                                          glucose syrup (g)                                                                         available                                                         Glucose syrup (h)                                                                      40.8                                                                             Pd/Bi/C according                                                                       3.20 1.85 95.5                                          with high DE                                                                              to example 1                                                      Glucose syrup (i)                                                                      38.0                                                                             Pd/Bi/C according                                                                       2.50 1.55 96.0                                          with medium DE                                                                            to example 1                                                      Glucose syrup (i)                                                                      38.0                                                                             Pd/C commercially                                                                       7.45 15.0 60.0                                          with medium DE                                                                            available                                                         Glucose syrup (j)                                                                      33.1                                                                             Pd/Bi/C according                                                                       3.00 1.70 94.9                                          with low DE to example 1                                                      Glucose syrup (j)                                                                      40.8                                                                             Pd/Bi/C according                                                                       3.05 2.50 92.5                                          with low DE to example 3                                                      Maltodextrin (m)                                                                       27.7                                                                             Pd/Bi/C according                                                                       2.20 2.30 92.0                                          with medium DE                                                                            to example 1                                                      __________________________________________________________________________     *fructose content 4.1%                                                   

The examination of the results recorded in table IV makes it possible tocome to the conclusions developed hereafter.

The catalytic oxidation of the polysaccharides by means of a Pd/Bi orPd/Pb catalyst on carbon makes it possible to reach a conversion rate ofthe polysaccharides of 92 to 97% for products whose D.E. value andglucidic distribution range within a very broad field. The oxidationperformed with these preferred catalysts is practically total and is notaffected by the presence of polysaccharides whose degree ofpolymerization is higher than 10. The low content of reducing sugars aswell as the High Performance Liquid Chromatography (HPLC) indicate thatall polysaccharides are oxidized and that the glucidic distribution isintegrally preserved.

In return, the tests carried out with a Pd/C catalyst commerciallyavailable indicate that the reaction time as well as the content ofreducing sugars significantly increase. The difference is all the moreimportant since the initial D.E. is low. This observation reflects thedifficulty in oxidizing polysaccharides whose degree of polymerizationis high.

More precisely, in the case of the catalytic oxidation of a glucosesyrup having a D.E. of medium value, with a Pd/C non-doped catalystcommercially available, the High Performance Liquid Chromatography(HPLC) shows that only the fraction constituted of glucose is totallyoxidized. The maltose fraction is oxidized to approximately 50%, themaltotriose fraction is oxidized to approximately 30% and the upperpolysaccharides undergo a minor oxidation only. The Pd/C catalystscommercially available are therefore relatively unsuitable for the totaloxidation of the polysaccharides.

We claim:
 1. A process for the preparation of polyhydroxycarboxylic acids by way of the selective oxidation of polysaccharides having a reducing terminal function of the aldose type, the said process comprising:selecting at least one polysaccharide having a reducing function of the aldose type in the form of an aqueous solution, dispersing into the said polysaccharide solution a catalyst based on a noble metal selected from the group consisting of palladium, platinum, rhodium and osmium and fixed on an inert support, said catalyst being "doped" with at least one metal, called promoter and selected from the group consisting of those of the Groups IV, V and VI of the Periodic Table, starting the reaction by supplying the polysaccharide solution, having dispersed therein the catalyst, with an oxygen containing gas and with an alkaline agent.
 2. Process according to claim 1, wherein the promoter is selected from the group consisting of bismuth, lead, antimony and selenium.
 3. Process according to claim 1, wherein the inert support is selected from the group consisting of carbon, alumina, silica, silica-alumina, barium sulfate and titanium oxide.
 4. Process according to claim 1, wherein the catalyst is constituted by at least one of the metals of the group consisting of palladium and platinum fixed on carbon and doped by at least one promoter of the group consisting of bismuth and lead.
 5. Process according to claim 1, wherein the content in the catalyst of the at least one noble metal of the group consisting of palladium and platinum, expressed in terms of metal, is comprised between 1 and 10 wt % with respect to the support.
 6. Process according to claim 1, wherein the content in the catalyst of the at least one metal of the group consisting of bismuth and lead, expressed in terms of metal, is comprised between 1 and 300 wt % with respect to the noble metal.
 7. Process according to claim 6, wherein the content in the catalyst of the at least one metal of the group consisting of bismuth and lead, expressed in terms of metal, is comprised between 5 and 10 wt % with respect to the noble metal.
 8. Process according to claim 1, wherein the aqueous solution of the at least one polysaccharide comprising a reducing function of the aldose type has a concentration comprised between 5 and 60 wt %.
 9. Process according to claim 1, wherein, in the aqueous solution of the at least one polysaccharide, the polysaccharide is selected from the group consisting of starch hydrolysates and glucose syrups, the concentration of the aqueous solution being comprised between 20 and 40 wt %.
 10. Process according to claim 1, wherein the temperature at which the reaction of oxidation is carried out, is comprised between 20° and 90° C., the reaction time being comprised between 30 minutes and 5 hours.
 11. Process according to claim 10, wherein the temperature to between 25° and 60° C.
 12. Process according to claim 1, wherein the pH of the aqueous solution of the at least one polysaccharide wherein is dispersed the catalyst, is maintained by means of at least one alkaline agent at a value comprised between 7.5 and 11.0.
 13. Process according to claim 12, wherein the pH of the aqueous solution is between 8.0 and 10.0.
 14. A process for the preparation of polyhydroxycarboxylic acids by way of the selective oxidation of polysaccharides having a reducing terminal function of the aldose type from the group consisting of starch hydrolysates and glucose syrups whose D.E. (dextrose equivalent) is comprised between 90 and 5, the said process comprising:selecting at least one starch hydrolysate or glucose syrup in the form of an aqueous solution, dispersing into the said solution of starch hydrolysate or glucose syrup a catalyst based on a noble metal selected from the group consisting of palladium, platinum, rhodium and osmium and fixed on an inert support, said catalyst being "doped" with at least one metal, called promoter and selected from the group consisting of those of the Groups IV, V and VI of the Periodic Table, starting the reaction by supplying the starch hydrolysate or glucose syrup solution, having dispersed therein the catalyst, with an oxygen containing gas and with an alkaline agent.
 15. Process according to claim 14, wherein the promoter is selected from the group consisting of bismuth, lead, antimony and selenium.
 16. Process according to claim 14, wherein the inert support is selected from the group consisting of carbon, alumina, silica, silica-alumina, barium sulfate and titanium oxide.
 17. Process according to claim 14, wherein the catalyst is constituted by at least one of the metals of the group consisting of palladium and platinum fixed on carbon and doped by at least one promoter of the group consisting of bismuth and lead.
 18. Process according to claim 14, wherein the content in the catalyst of the at least one noble metal of the group consisting of palladium and platinum, expressed in terms of metal, is comprised between 1 and 10 wt % with respect to the support.
 19. Process according to claim 14, wherein the content in the catalyst of the at least one metal of the group consisting of bismuth and lead, expressed in terms of metal, is comprised between 1 and 300 wt % with respect to the noble metal.
 20. Process according to claim 19, wherein the content in the catalyst of the at least one metal of the group consisting of bismuth and lead, expressed in terms of metal, is comprised between 5 and 10 wt % with respect to the noble metal.
 21. Process according to claim 14, wherein the aqueous solution of the at least one starch hydrolysate and glucose syrup has a concentration comprised between 20 and 40 wt %.
 22. Process according to claim 14, wherein the starch hydrolysates and glucose syrups have a D.E. (dextrose equivalent) comprised between 85 and
 15. 23. Process according to claim 22, wherein the starch hydrolysates and glucose syrups have a D.E. (dextrose equivalent) comprised between 75 and
 15. 24. Process according to claim 14, wherein the amount of catalyst dispersed in the aqueous solution of the at least one starch hydrolysate or glucose syrup is such that the concentration in noble metal of the group consisting of palladium and platinum, expressed in terms of metal, is comprised between 0.005 and 1 wt % with respect to the amount of starch hydrolysate or glucose syrup.
 25. Process according to claim 24, wherein the amount of catalyst dispersed in the aqueous solution of the at least one starch hydrolysate or glucose syrup is such that the concentration of noble metal of the group consisting of palladium and platinum, expressed in terms of metal, is comprised between 0.01 and 0.4 wt % with respect to the amount of starch hydrolysate or glucose syrup.
 26. Process according to claim 14, wherein the alkaline agent is selected from the group consisting of calcium hydroxide, lithium hydroxide, magnesium hydroxide, zinc carbonate and manganese carbonate. 